Wednesday, September 25, 2013

My blog has now just crossed 200,000 views! I am extremely
happy that it’s gotten this many views and that I’ve been able to reach out to
so many people from all across the world. I love writing tutorials and articles
about microcontrollers, power electronics, SMPS and everything electronics! I
love it even more when my writing reaches out to people from all over the world
to help them learn something new or to find the solution to some problem! So, I
say thank you to all my readers and followers and hope that I have been able to
help you and that I can continue helping you!

If you have any topic in mind that you want me to write
about, please mention that in the comments. While I cannot promise you that I
will write up on it, I will certainly try.

University has started (if you’re wondering why the title
says Ithaca, that’s because Cornell University is located in Ithaca, New York,
USA) and it’s been a month. The work load is quite high, so I may take quite a
while to write some new article or tutorial or even to answer to your comments.
So, please be patient.

On that note, I’m doing an independent study under the
supervision of Professor Bruce Land here at Cornell University. My project is to
explore the different peripherals on the two microcontrollers: AT91SAM3X8E (ARM
Cortex M3) and the PIC32MX250F128B (DMIPS), write up documentation on the
different peripherals and then compare and contrast the two microcontrollers in
terms of peripheral functionality, ease of learning, ease of programming and
ease of use.

So, in the coming days, I will post quite a lot of articles
or tutorials based on these two microcontrollers and their peripherals.

If you’re interested, here are the links to the datasheets
of the two microcontrollers:

For now, this is it. Again, if you want me to write up on
any topic, please mention that in the comments section. If you have any ideas,
suggestions or feedback for my blog, please let me know in your comments! Till
then, enjoy the blog and I hope it helps you in your projects and learning
endeavors.

The
photo above clearly illustrates phase angle control: output voltage
controlled by the gate drive signal applied to a thyristor. What is
phase angle control? That is what I'm going to talk about in this
article.

Phase angle control is a method of PWM applied to AC input
voltages, usually the mains supply. Of course, the AC supply could be from a
transformer or any other AC source, but the mains supply is the most common
input – this gives the phase angle control method its greatest usefulness. It
has of course become quite obvious from the title (and I’m sure most of you
reading will already know this) that the purpose of phase angle control is to
control or limit power to the load.

The power device used in phase angle controllers is a
thyristor – mostly triacs or SCRs. (There are methods of phase controlling
employing high frequency switching using a MOSFET or IGBT, but here I’ll talk
about phase angle control with thyristors only). The power flow to the load is
controlled by delaying the firing angle (firing time each half-cycle) to the
power device.

We know that the thyristor is a latching device – when the
thyristor is turned on by a gating signal and the current is higher than the
holding current and the latching current, the thyristor stays on, until the
current through it becomes sufficiently low (very close to zero). The thyristor
turns off when current through it becomes zero, as happens at the AC mains zero
crossing. This is the natural line commutation. (Another method of turning the
thyristor off is by forced commutation. I won’t go into that now.) The
assumption here is that the load is resistive and has little to no inductance.
Of course, this is not always the case, as inductive loads are often used.
However, I’ll work with this assumption for now.

Now, with that covered, you should read this article first
before proceeding to the rest of this article:

I’ve added the circuit, code and simulation of an example
later in this article. And that uses a triac as the power device. So, from now
on, I’ll just refer to the triac instead of talking about a thyristor in
general.

So, in phase angle control, a gate pulse is sent to the
triac. This is sent at a time between one zero crossing and the next. Without
the gate pulse sent to the triac, right after zero-crossing, the triac is off
and no current flows through it. After a certain time, the gating signal is
given to the triac and it turns on. The triac then stays on until the current
through it becomes zero (natural line commutation). This is at the next zero
crossing. For simplicity’s sake and as usually should be, assume that the
current through the triac (when on) is larger than the latching current and the
holding current. If you didn’t already know this, the latching current is the
current that must pass through the triac right after it is turned on to ensure
that it latches. The holding current is the current level through the triac
below which the triac will turn off. So, the assumption that current through
the triac is higher than the latching current and the holding current means
that the triac stays on once it is fired on. It stays on until the current
through it is zero.

This means that the voltage is supplied to the load for a
fraction of the cycle, determined by how long the triac is on. How long the
triac is on, is, in turn, determined by the delay time between the
zero-crossing and the applying of the triac gating signal.

So, to sum it up, we adjust the voltage or power delivered to
the load by delaying the trigger signal to the triac. One thing to remember is
that, the delivered voltage and power are not linearly related to the firing
phase angle.

There are two voltages here that we are concerned with – the
RMS voltage and the average voltage. The RMS voltage governs the power output
to resistive loads such as incandescent bulbs and resistive heaters. The
average value relates to devices that function on the average voltage level. This
is important because, when testing, your voltmeter will register the average
voltage – and not the true RMS voltage – unless you have a “true RMS
voltmeter”. Most inexpensive voltmeters are not true RMS meters but will
respond to average value changes.

To clarify why power and voltage are not linearly related,
let’s examine the formula relating the two.

So, assuming a constant resistance (be
careful if you’re using incandescent lamps, since they are NOT constant
resistance devices), power is directly proportional to the square of the
voltage. So, if you half the voltage, the power is not halved, but is reduced to
one-fourth the original power! One-fourth power with half the voltage!

Now let’s now go on to the design part – how we’re actually
going to do this.

For
the microcontroller, I’ve chosen the extremely popular ATmega8. However,
since this application requires only a few pins, you can easily use any other
small microcontroller for this purpose, such as ATtiny13.

The
zero-crossing is done using the bridge-optocoupler method as I had previously
shown. For details regarding the zero-crossing, please go through the article:

Take a look at the circuit diagram below. PORTB is connected to a DIP-switch.
Let me first talk about the DIP-switch. The DIP-switch I chose has 3
individual switches, each of which can take on or off position. So, the
DIP-switch can have 8 possible states and we can assign a digital value
to the DIP-switch states. Let's assume that switch 0 is the switch connected to PORTB0/PINB0,
that switch 1 is the switch connected to PORTB1/PINB1, and that switch 2 is the switch connected to PORTB2/PINB2. So,
the possible DIP-switch states with their assigned values are:

So, the DIP-switch can alter the values of PORTB0, PORTB1 and PORTB2. I made all of PORTB input pins and enabled the pull-ups. So, by default, all PORTB pins are high. When one of the switches is ON or shorted, that corresponding pin is low. So, only PORTB0, PORTB1 and PORTB2 can be switched to be high or low. The other pins will be high at all times. So, in order to read the value of the DIP-switch, I first inverted each individual bit of PORTB when reading (PINB) and then added 1 to the value as I wanted one to be the minimum value. Why one the minimum value? If you see the code later, you will see that the register "Count" is used to determine the delay time before the triac is fired. Count will have the value of the DIP-switch state plus one. I wanted a minimum delay before the triac is fired so that there is always a delay after the zero-crossing after the triac is fired. So, the function of the DIP-switch is now quite obvious. The DIP-switch sets the time after the zero-crossing after which the triac is fired. When only PORTB0/PINB0 is low, the delay time before firing the triac is minimum; it is equal to 800µs. When PORTB0/PINB0, PORTB1/PINB1 and PORTB2/PINB2 are all low, the delay time before firing the triac is maximum; it is equal to 800µs * 8 = 6.4ms. The adjusting delay time before firing adjusts the firing angle. How the time is related to the firing angle can be seen at the bottom of this article. The formula and an example are provided.

So, the program is basically as follows:

The zero-crossing is first checked.
After zero-crossing occurs, a small delay is present before the triac is fired. This delay is set by the value of the DIP-switch. So, the triac is fired a while after the zero-crossing occurs.
The gating signal is removed 250µs after that. 250µs
is enough time to ensure that the triac has turned on. Even though the
gating
signal is removed, the triac stays on until the next zero-crossing as it
is a
latching device. Now you may ask, why remove the gating signal? Just
keep it on till the next zero-crossing. Well, that'd work too. The
problem there would be that, there would be high switching losses of the
thyristor. The gate drive resistance would dissipate immense amounts of
power - all for no reason, since the triac would be on even if the
signal was removed.

In my circuit, I used the internal 4MHz RC oscillator. So, make sure you set the fuse bits correctly if you're using the internal oscillator.

The rest of the code should be easy to understand and should
be self-explanatory – I’ve added comments to help you understand.

Now let’s take a look at my circuit setup and then the different output waveforms using
this code:

Fig. 1 - Circuit Diagram (Click on image to enlarge)

You should choose R1 depending on the gate current
requirements of the triac. It must also have a sufficiently high power
dissipation rating. Usually, the instantaneous power may be very high. But
since current flows through the resistor for only 250us (1/40 of a 50Hz half cycle),
the average power is small enough. Usually, 2W resistors should suffice.

Let’s assume we’re using a BT139-600 triac. The maximum required trigger
current is 35mA. Although the typical trigger current is lower, we should
consider the maximum required trigger current. This is 35mA for quadrants I, II
and III. We will only be firing in quadrants I and III. So, that is ok for us –
we need to consider 35mA current.

If you aren’t sure what quadrants are, here’s a short
description. First take a look at this diagram:

Fig. 2 - Triac Triggering Quadrants

If you look back again at the diagram, you’ll see that we’re
driving gate from MT2. So, we can say that, with respect to MT1, when MT2 is
positive, so is the gate. With respect to MT1, when MT2 is negative, so is the
gate. From the diagram above, you can see that these two cases are in quadrants
I and III. This is what I meant when I mentioned that we’re driving only in
quadrants I and III.

The driver in the circuit is the MOC3021. This is a random
phase optically isolated triac output driver. When the LED is turned on, the
triac in the MOC3021 turns on and drives the main triac in the circuit. It is a
“random phase” driver meaning that it can be driven on at any time during the
drive signal, as is required for phase angle control. There are other drivers
that only allow drive at the zero-crossing. These cannot be used for phase
angle control as phase angle control requires drive after zero-crossing. For
guaranteeing that the triac is latched, the LED side of the MOC3021 must be
driven with at least 15mA current. The maximum current rating for the LED is
60mA. The peak current rating for the triac is 1A. You should find that we have
stayed within these limits in the design.

You can clearly see that before the gate driving signal is
applied, there is no output (illustrated by the flat yellow line).When the gate
driving signal is applied, the triac turns on. There is an output and the triac
stays on till the next zero crossing. After this again, there is no output till
the next gate drive signal is applied.

Here, the gate is driven 4.8ms after the zero-crossing (PORTB2/PINB2 and PORTB0/PINB0 low : DIP-switch state = 5):

Fig. 8 - Triac firing with 4.8 ms delay

Green: Input AC

Yellow: AC Output after phase angle control
Pink: Gate Drive signal

Here, the gate is driven 5.6ms after the zero-crossing (PORTB2/PINB2 and PORTB1/PINB1 low : DIP-switch state = 6):

Fig. 9 - Triac firing with 5.6 ms delay

Green: Input AC

Yellow: AC Output after phase angle control
Pink: Gate Drive signal

Here, the gate is driven 6.4ms after the zero-crossing (PORTB2/PINB2, PORTB1/PINB1 and PORTB0/PINB0 all low : DIP-switch state = 7):

Fig. 10 - Triac firing with 6.4 ms delay

Green: Input AC

Yellow: AC Output after phase angle control
Pink: Gate Drive signal

Now, to finish things off, I’ll show you how to find the RMS
value of the output voltage.

We first need to know how to relate the firing delay with
firing angle. We know that one complete sine wave is 360°. That is 2π
radians. We then need to know that the firing angle α = ωt, where ω = 2πf.
Since, we’re working with 50Hz here, f=50Hz. Thus, ω = 100π.
Just to test this relationship, let’s use t = 0.020 seconds (20ms). Thus α = 100π *
0.020 = 2π, as told before.

So, if we are firing after 4ms, (α = 0.4 π),
the output RMS voltage is:

Remember,
at the beginning, I mentioned that the voltage output is not linearly
correlated with the firing angle? This is what I meant. Here, the delay
is 4ms. So, the triac is on for 60% of the cycle. But the output RMS
voltage is 183.2V - 83% of the input voltage. The lack of direct
proportionality is evident here. The reason behind this is the shape of
the AC - sinusoidal.

Now, I give you the task of finding the
RMS voltage for the other cases mentioned in this tutorial.

If you want to then find power, you can
use the relationship P = V2/R to find the power. The assumption here
is that the resistance is constant, as was assumed at the beginning of the
tutorial. If the resistance is not constant, power will still vary will
resistance, just not directly proportionally.

Here in this article, I’ve talked about
phase angle control with some background information on triacs. I’ve shown how
to implement phase angle control with an AVR and also how to calculate the RMS
voltage of the output. I hope I’ve been able to explain this extremely important
topic to you clearly and hope that you can now successfully build your own
power control circuits using phase angle control with triacs.

Reference Book:
One
of the best books for understanding the theory behind phase angle
control is "POWER ELECTRONICS - CIRCUITS, DEVICES AND APPLICATIONS" by
Muhammad H. Rashid. If you want to learn more about thyristors or phase
angle control, I recommend reading this book for more info.

Saturday, June 29, 2013

Pulse skipping modulation (PSM) or cycle control or burst fire is a method
of power control where whole cycles of voltage are applied to the load. Here I’ll
talk about PSM involving a thyristor, specifically a triac. The triac connects
the AC supply to the load for a given number of cycles and then disconnects the
AC supply for another given number of cycles. It has of course become quite
obvious from the title that the purpose of PSM is to control or limit power to
the load.

We know that the thyristor is a latching device – when the
thyristor is turned on by a gating signal and the current is higher than the
holding current and the latching current, the thyristor stays on, until the
current through it becomes sufficiently low (very close to zero). The thyristor
turns off when current through it becomes zero, as happens at the AC mains zero
crossing. This is the natural line commutation. (Another method of turning the
thyristor off is by forced commutation. I won’t go into that now.) The
assumption here is that the load is resistive and has little to no inductance.
Of course, this is not always the case, as inductive loads are often used. However,
I’ll work with this assumption for now.

Now, with that covered, you should read this article first
before proceeding to the rest of this article:

I’ve added the circuit, code and simulation of an example
later in this article. And that uses a triac as the power device. So, from now on,
I’ll just refer to the triac instead of talking about a thyristor in general.

So, in pulse skipping modulation (PSM) or cycle control, the
load receives power for a number of cycles, during which time the triac is on,
and does not receive power for another number of cycles, during which time the
triac is off. When the triac is to be turned on, the gating signal to the triac
is given right after zero-crossing. The triac then stays on until the current
through it becomes zero (natural line commutation). This is at the next zero
crossing. For simplicity’s sake and as usually should be, assume that the
current through the triac (when on) is larger than the latching current and the
holding current. If you didn’t already know this, the latching current is the
current that must pass through the triac right after it is turned on to ensure
that it latches. The holding current is the current level through the triac
below which the triac will turn off. So, the assumption that current through
the triac is higher than the latching current and the holding current means
that the triac stays on once it is fired on. It stays on until the current
through it is zero.

Let’s say that we are going to control over 5 complete
cycles, that is, 10 half-cycles. My AC input voltage is 220V RMS, is sinusoidal
in shape and has 50Hz frequency.

So, the principle is that we are going to supply voltage to
the load for a certain number of cycles. Let’s say we’ll supply voltage for 3
cycles and will shut supply to the load off for 2 cycles. This is a duty cycle
of 60%. The duty cycle is defined as the ratio (expressed in percentage) of the
number of on cycles to the total number of cycles we are controlling.

The output RMS voltage is related to the supply voltage by
the relationship:

whereVout
= Output RMS voltageVs = Supply RMS Voltagek = duty cycle

The output is related to the voltage by
the relationship P = V2/R. So, assuming a constant resistance, power is directly proportional to the square of the voltage. So, if
you half the voltage, the power is not halved, but is reduced to one-fourth the
original power! One-fourth power with half the voltage! But the general idea is
that, by controlling the output voltage, you can control the output power.

So, using the previous values, where we
have a duty cycle of 60% and a 220V RMS input voltage, the RMS output voltage
is:

Now let’s consider the ratio of the powers.
P = V2/R. Therefore, by using pulse skipping modulation, the power
changed by changing the voltage is related by:

This means that if you had a resistance R
as load, and if the power was P when the voltage supplied was Vs, using PSM
with a duty cycle of 0.6 decreased the power to 0.6P – using PSM we changed the
output voltage and thus output power.

Now let’s now go on to the design part – how we’re actually
going to do this.

For
the microcontroller, I’ve chosen the extremely popular PIC 16F877A. However,
since this application requires only a few pins, you can easily use any other
small microcontroller for this purpose, such as PIC 12F675.

The
zero-crossing is done using the bridge-optocoupler method as I had previously
shown. For details regarding the zero-crossing, please go through the article:

The total number of half-cycles I want to control is stored
in the register TotalCount. Count holds the number of half-cycles I want
voltage supplied to the load. So, the ratio of Count to TotalCount equals the
duty cycle.

Each time the triac is fired, the value of nCount is
decremented, until it reaches zero. When nCount equals zero, the triac is no
longer fired. At the same time, the register nTotalCount is decremented each
zero-crossing. So it keeps track of the number of half-cycles elapsed. Initially
nTotalCount is loaded with the value of TotalCount so that nTotalCount keeps
track of the number of half-cycles we want to control. After that number of
half-cycles elapses, nTotalCount is reloaded with the value of TotalCount and
nCount is reloaded with the value of Count. Pulse skipping modulation starts
over again.

In our code, we want to control 10 half-cycles, which means 5 cycles. We want
voltage to be supplied to the load for 3 cycles and off for 2 cycles. This is
done, as nCount counts down from 6 to 0 while firing the triac, firing the
triac 6 consecutive half-cycles. For the next 4 half-cycles, while nTotalCount
is still counting down to zero, the triac is off. So, voltage is not supplied
to the load. The duty cycle is 60%.

The triac is fired right after zero-crossing occurs. One advantage
of this is that very little radio frequency interference is produced. However,
care must be taken that an unequal number of positive and negative half-cycles
must not be present. This will introduce a DC component that can damage motors
and other devices. Cycle control also cannot be used with incandescent bulbs
because of high unacceptable flickering.

This problem with the DC component is avoided by ensuring
that equal numbers of positive and negative half-cycles are present. This is
done by ensuring that the Count and TotalCount (in the code above) are both
even numbers.

Now let’s take a look at my circuit setup and then the
output waveform using this code:

Fig. 1 - Circuit Diagram (Click on image to enlarge)

You should choose R1 depending on the gate current
requirements of the triac. It must also have a sufficiently high power
dissipation rating. Usually, the instantaneous power may be very high. But
since current flows through the resistor for only 250us (1/40 of a 50Hz half
cycle), the average power is small enough. Usually, 2W resistors should suffice.

Let’s assume we’re using a BT139-600 triac. The maximum required trigger
current is 35mA. Although the typical trigger current is lower, we should
consider the maximum required trigger current. This is 35mA for quadrants I, II
and III. We will only be firing in quadrants I and III. So, that is ok for us –
we need to consider 35mA current.

If you aren’t sure what quadrants are, here’s a short
description. First take a look at this diagram:

Fig. 2 - Triac Triggering Quadrants

If you look back again at the diagram, you’ll see that we’re
driving gate from MT2. So, we can say that, with respect to MT1, when MT2 is
positive, so is the gate. With respect to MT1, when MT2 is negative, so is the
gate. From the diagram above, you can see that these two cases are in quadrants
I and III. This is what I meant when I mentioned that we’re driving only in
quadrants I and III.

The driver in the circuit is the MOC3021. This is a random
phase optically isolated triac driver output. When the LED is turned on, the
triac in the MOC3021 turns on and drives the main triac in the circuit. It is a
“random phase” driver meaning that it can be driven on at any time during the
drive signal. There are other drivers that only allow drive at the
zero-crossing. These can also be used for PSM as the triac is only driven at
zero-crossing. For guaranteeing that the triac is latched, the LED side of the
MOC3021 must be driven with at least 15mA current. The maximum current rating
for the LED is 60mA. The peak current rating for the triac is 1A. You should
find that we have stayed within these limits in the design.

You can clearly see that out of every 5 complete cycles, the
output consists of 3 complete cycles and 2 are missing – these are when the
triac is off. Notice that the triac is fired 6 times in every 5 complete cycles
– the triac is on for 6 half-cycles and off for the next 4 half-cycles.

Now, let’s look at a few other cases.

Here’s the output with 3 complete cycles on out of every 4
complete cycles:

Here in this article, I’ve talked about pulse
skipping modulation – also known as cycle control or burst fire – with some background
information on triacs. I’ve shown how to implement pulse skipping modulation / cycle
control / burst fire with a PIC and also how to calculate the RMS voltage of the output. I
hope I’ve been able to explain this important topic to you clearly and hope
that you can now successfully build your own power control circuits using pulse
skipping modulation (or cycle control or burst fire) with triacs.

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About Me

I am Syed Tahmid Mahbub, from Dhaka, Bangladesh, born on August 1, 1994.
Electronics is my passion and from class V, I have been learning electronics. I learnt and worked mostly on SMPS, power electronics, microcontrollers and integration of microcontrollers with SMPS and power electronics. I've used PIC and AVR microcontrollers - PIC 10F, 12F, 16F, 18F, 24F, dsPIC 30F, 33F, PIC32, ATmega and ATtiny, integrating them with various SMPS and power electronics circuits.
I have completed my Bachelor's degree from Cornell University (Class of 2017) in Ithaca, New York, USA, majoring in Electrical and Computer Engineering (ECE).
I am a member of the forum www.edaboard.com, where I am an "Advanced Member Level 5" (the highest level attainable) and also the forum allaboutcircuits.com, where I am a "Senior Member". I post to help solve electronics-related problems of engineers and engineering students from all over the world.
I love watching and playing cricket and football (soccer), and listening to music.
I am now a hardware engineer at Apple in Silicon Valley, California, USA.